Serge Lang Linear Algebra Third Edition - Página de Dibene

308 Pages·2009·10.67 MB·English

Undergraduate Texts in Mathematics
Serge Lang
Linear
Algebra
Third Edition
Springer
Undergraduate Texts in Mathematics
Editors
s.
Axler
F. W. Gehring
K. A. Ribet
Springer
New York
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Serge Lang
Linear Alge bra
Third Edition
With 21 Illustrations
Springer
Serge Lang
Department of Mathematics
Yale University
New Haven, CT 06520
USA
Editorial Board
S. Axler F.W. Gehring K.A. Ribet
Mathematics Department Mathematics Department Mathematics Department
San Francisco State East Hall University of California,
University University of Michigan at Berkeley
San Francisco, CA 94132 Ann Arbor, MI 48109 Berkeley, CA 94720-3840
USA USA USA
Mathematics Subject Classification (2000): IS-0 1
Library of Congress Cataloging-in-Publication Data
Lang, Serge
Linear algebra.
(Undergraduate texts in mathematics)
Includes bibliographical references and index.
I. Algebras, Linear. II. Title. III. Series.
QA2Sl. L.26 1987 SI2'.S 86-21943
ISBN 0-387 -96412-6 Printed on acid-free paper.
The first edition of this book appeared under the title Introduction to Linear Algebra © 1970
by Addison-Wesley, Reading, MA. The second edition appeared under the title Linear Algebra
© 1971 by Addison-Wesley, Reading, MA.
© 1987 Springer-Verlag New York, Inc.
All rights reserved. This work may not be translated or copied in whole or in part without the written
permission of the publisher (Springer-Verlag New York, Inc., 17S Fifth Avenue, New York, NY
10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use In
connection with any form of information storage and retrieval, electronic adaptation, computer
software, or by similar or dissimilar methodology now known or hereafter developed is forbidden.
The use in this publication of trade names, trademarks, service marks, and similar terms, even if they
are not identified as such, is not to be taken as an expression of opinion as to whether or not they are
subject to proprietary rights.
Printed in the United States of America.
19 18 17 16 IS 14 13 12 11 (Corrected printing, 2004) SPIN 10972434
Springer-Verlag is part of Springer Science+Business Media
springeronline. com
Foreword
The present book is meant as a text for a course in linear algebra, at the
undergraduate level in the upper division.
My Introduction to Linear Algebra provides a text for beginning stu
dents, at the same level as introductory calculus courses. The present
book is meant to serve at the next level, essentially for a second course
in linear algebra, where the emphasis is on the various structure
theorems: eigenvalues and eigenvectors (which at best could occur only
rapidly at the end of the introductory course); symmetric, hermitian and
unitary operators, as well as their spectral theorem (diagonalization);
triangulation of matrices and linear maps; Jordan canonical form; convex
sets and the Krein-Milman theorem. One chapter also provides a com
plete theory of the basic properties of determinants. Only a partial treat
ment could be given in the introductory text. Of course, some parts of
this chapter can still be omitted in a given course.
The chapter of convex sets is included because it contains basic results
of linear algebra used in many applications and "geometric" linear
algebra. Because logically it uses results from elementary analysis (like a
continuous function on a closed bounded set has a maximum) I put it at
the end. If such results are known to a class, the chapter can be covered
much earlier, for instance after knowing the definition of a linear map.
I hope that the present book can be used for a one-term course. The
first six chapters review some of the basic notions. I looked for effi
ciency. Thus the theorem that m homogeneous linear equations in n
unknowns has a non-trivial soluton if n > m is deduced from the dimen
sion theorem rather than the other way around as in the introductory
text. And the proof that two bases have the same number of elements
(i.e. that dimension is defined) is done rapidly by the "interchange"
VI FOREWORD
method. I have also omitted a discussion of elementary matrices, and
Gauss elimination, which are thoroughly covered in my Introduction to
Linear Algebra. Hence the first part of the present book is not a substi
tute for the introductory text. It is only meant to make the present book
self contained, with a relatively quick treatment of the more basic mate
rial, and with the emphasis on the more advanced chapters. Today's
curriculum is set up in such a way that most students, if not all, will
have taken an introductory one-term course whose emphasis is on
matrix manipulation. Hence a second course must be directed toward
the structure theorems.
Appendix 1 gives the definition and basic properties of the complex
numbers. This includes the algebraic closure. The proof of course must
take for granted some elementary facts of analysis, but no theory of
complex variables is used.
Appendix 2 treats the Iwasawa decomposition, in a topic where the
group theoretic aspects begin to intermingle seriously with the purely linear
algebra aspects. This appendix could (should?) also be treated in the
general undergraduate algebra course.
Although from the start I take vector spaces over fields which are
subfields of the complex numbers, this is done for convenience, and to
avoid drawn out foundations. Instructors can emphasize as they wish
that only the basic properties of addition, multiplication, and division are
used throughout, with the important exception, of course, of those theor
ies which depend on a positive definite scalar product. In such cases, the
real and complex numbers play an essential role.
New Haven, SERGE LANG
Connecticut
Acknowledgments
I thank Ron Infante and Peter Pappas for assisting with the proof reading
and for useful suggestions and corrections. I also thank Gimli Khazad for
his corrections.
S.L.
Contents
CHAPTER I
Vector Spaces 1
§1. Definitions .. 2
§2. Bases. . .. . .... 10
§3. Dimension of a Vector Space . 15
§4. Sums and Direct Sums . . . . . 19
CHAPTER II
Matrices . . 23
§1. The Space of Matrices ..... 23
§2. Linear Equations. . . . 29
§3. Multiplication of Matrices . 31
CHAPTER III
Linear Mappings . 43
§ 1. Mappings . . . 43
§2. Linear Mappings. . 51
§3. The Kernel and Image of a Linear Map 59
§4. Composition and Inverse of Linear Mappings . . 66
§5. Geometric Applications. . . . . . . . . . . . . . . . 72
CHAPTER IV
Linear Maps and Matrices. . . . . . . . . . . . . 81
§1. The Linear Map Associated with a Matrix. . 81
§2. The Matrix Associated with a Linear Map. 82
§3. Bases, Matrices, and Linear Maps . . . . . . . 87
Vl11 CONTENTS
CHAPTER V
Scalar Products and Orthogonality. 95
§1 . Scalar Products. . . . . . . . . . . 95
§2. Orthogonal Bases, Positive Definite Case .. 103
§3. Application to Linear Equations; the Rank .. 113
§4. Bilinear Maps and Matrices . . . . . . 118
§5. General Orthogonal Bases ....... . 123
§6. The Dual Space and Scalar Products 125
§7. Quadratic Forms .............. . 132
§8. Sylvester's Theorem . . . . . . . . . . . . 135
CHAPTER VI
Determinants ..... 140
§1. Determinants of Order 2 .. 140
§2. Existence of Determinants 143
§3. Additional Properties of Determinants. 150
§4. Cramer's Rule . . . . . . . . . . . . . . . . 157
§5. Triangulation of a Matrix by Column Operations 161
§6. Permutations . . . . . . . . . . . . . . . . . . . . . . . 163
§7. Expansion Formula and Uniqueness of Determinants 168
§8. Inverse of a Matrix . . . . . . . . . . . . . . . . . 174
§9. The Rank of a Matrix and Subdeterminants . . . . .. 177
CHAPTER VII
- Symmetric, Hermitian, and Unitary Operators. . 180
§1. Symmetric Operators 180
§2. Hermitian Operators 184
§3. Unitary Operators . . 188
CHAPTER VIII
Eigenvectors and Eigenvalues 194
§1. Eigenvectors and Eigenvalues . 194
§2. The Characteristic Polynomial. . 200
§3. Eigenvalues and Eigenvectors of Symmetric Matrices 213
§4. Diagonalization of a Symmetric Linear Map. . 218
§5. The Hermitian Case. . . . . . . . . . . . 225
§6. Unitary Operators . . . . . . . . . . . . . . . . . 227
CHAPTER IX
Polynomials and Matrices . 231
§1. Polynomials. . . . . . . . . . . . . . . . . . . . 231
§2. Polynomials of Matrices and Linear Maps . . 233
CONTENTS IX
CHAPTER X
Triangulation of Matrices and Linear Maps 237
§1. Existence of Triangulation . . . . . . 237
§2. Theorem of Hamilton-Cayley ... 240
§3. Diagonalization of Unitary Maps. 242
CHAPTER XI
Polynomials and Primary Decomposition. . 245
§1. The Euclidean Algorithm .. 245
§2. Greatest Common Divisor . . . . . . . . . . . . . . 248
§3. Unique Factorization ........... . 251
§4. Application to the Decomposition of a Vector Space. 255
§5. Schur's Lemma. . . . . . . . 260
§6. The Jordan Normal Form ................ . 262
CHAPTER XII
Convex Sets 268
§1 . Definitions ....... . 268
§2. Separating Hyperplanes. 270
§3. Extreme Points and Supporting Hyperplanes 272
§4. The Krein-Milman Theorem .......... . 274
APPENDIX I
Complex Numbers............................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
APPENDIX II
Iwasawa Decomposition and Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
Index..................................................................... 293